1. Field of the Invention
The present invention relates to optical disk drive systems, and more particularly, to a system which corrects for disturbances of a tracking error signal caused by pre-formatted sector information on a magneto-optical disk.
2. Description of the Related Art
Optical data drive systems, including compact audio disk players and computer data storage devices, employ collimated light, such as a laser beam, to write to and/or read from a rotating optical disk which is capable of storing great amounts of data. The data is accessed by focusing a laser beam onto the disk and then detecting the information encoded in the reflected light beam. Various kinds of optical systems are known, including read only memory (ROM), write once read many (WORM), and erasable. The erasable systems include magneto-optic (MO) and phase change recording media.
In an optical disk drive system, data is generally stored in either of two configurations. In one configuration, data is stored on a series of concentric tracks formed on an optical disk. Random access to a particular area of the disk (and thus to particular data), is accomplished by scanning the laser beam radially across the tracks on the disk so that the desired track is located, and then scanning along the track until a particular area or sector of interest on the disk may be interrogated by the laser beam. In the second configuration, data is stored on a single, spiral track. Random access to the spiral track is accomplished in a manner similar to that for circular tracks. The laser beam scans radially across the boundaries of adjacent spiral track revolutions to locate a desired revolution until the particular area, or sector of interest on the disk is reached.
The laser beam must be properly focused on the disk while simultaneously following the data tracks. The focus is maintained by moving an objective lens through which the laser beam is directed relative to the disk using a focus actuator. The tracking position is adjusted by moving an optical head over the rotating optical disk, from which the laser beam is generated, in a radial direction with respect to the tracks on the disk using a linear actuator. The optical head typically also contains the focus lens, a focus actuator and a beam bender mirror. Various focus and tracking servo schemes are known in the art.
One of the most widely used track servo schemes uses tracking grooves. The data track regions are also referred to as lands. The tracking grooves are molded into the disk on either side of each data track. Grooves may be V-shaped or trapezoidal shaped. The focused laser beam has a Gaussian diameter comparable to the land width and is directed to the grooved structure of the disk. Diffraction of the laser beam caused by the grooves produces different intensity distributions between the zero, and plus or minus first diffraction orders depending upon the radial position of the beam spot with respect to the grooves. Generally, more intensity is diffracted into the first orders as the beam moves away from a land section and toward the groove section. These light intensity distributions are imaged onto a servo controlled optical detector and are used to generate a tracking error signal (TES) which in turn controls the linear actuator that positions the optical head in order to keep the beam on track. The same optical detector is commonly also used to generate a focus error signal (FES). The FES is used to control the focus actuator to maintain the light beam in focus on the disk.
A problem develops when an optical disk drive performs a seeking operation in order to scan, or interrogate a specific sector of a particular track. Disturbances of the tracking error signal (TES) caused by pre-formatted sector information, or hard sectors can cause seeking errors. The internal electronic "seek" circuitry of the drive relies upon an accurate count of each track crossed as the objective lens scans across the rotating optical disk. The tracking error signal from an optical disk in the absence of pre-formatted sector information resembles a sine wave. Each zero crossing of the sine wave like TES signal represents a crossing of either the center of a land or groove area of the track feature. The electronic tracking circuitry relies upon the zero crossings to accurately determine the number of data tracks traversed during the time the tracking error loop is operating in the open loop mode. Similarly, when the "seek" operation is being completed and the tracking servo begins closed loop operation, the electronic tracking circuitry "locks"on the zero crossing of the TES signal. The TES signal is corrupted when the focused spot of the laser beam interrogates a hard sector region on the optical disk, causing the voltage amplitude of the TES signal to inappropriately go to zero. When the optical disk drive system is operating in a "seek" mode, such inappropriate zero value of the TES signal can cause the track counter to incorrectly count the number of tracks traversed, resulting in a "seek" error. The disturbance of the TES signal can also cause the seek circuitry to begin closed loop operations at the wrong time relative to the zero crossing, which may also result in a seek error. The net effect is a reduction in "seek" performance and reliability.
Therefore, a need exists for a system which can overcome the problem of seeking errors caused by disturbances of the tracking error signal resulting from detection of pre-formatted sector information in an optical disk drive system.